Oil and gas operations involve accessing underground hydrocarbon reservoirs contained within subterranean formations. As part of ordinary operations, drilling is conducted to form a borehole in order to access desired sites. Once a borehole is formed, ordinarily a casing is provided within at least a portion of the borehole in order to define and stabilize the borehole for subsequent drilling or hydrocarbon retrieval. The borehole casing is commonly formed by steel piping set in place with the introduction of cement between the borehole wall and the piping. Maintaining the integrity of the casing and borehole is desirable during drilling and well completion, and the casing and borehole must withstand attack by borehole fluids as well as the high temperatures and pressures that are present downhole. The casing affords protection against various borehole fluids or other contaminates from entering the borehole or moving to unwanted zones. For example, proper protection by the casing can prevent accumulation of fluids in undesirable locations or zones, or flow of fluids from one zone to another, or to the surface. Other dangers include movement of fluids to contaminate the water table or creation of hazardous conditions such as explosive mixtures.
Accordingly, the integrity of the casing and borehole can be monitored during operations to prevent the occurrence of adverse conditions, including the presence of leaks, or the movement of fluids. Detection and discovery of the presence of leaks in the casing and borehole has been the focus of inquiry. A number of techniques are known for determining whether a well or a borehole has proper integrity and no leaks.